Triumph Repair: 76 Spitfire detonation, oil pressure gauge, spitbits

Question
I have a 76 Spitfire 1500. I am having a problem with "run-on" or detonation after the car is shut down.  I adjusted the timing to 2 ATDC.  The car runs great and has plenty of power.  However, when I shift (while the clutch is pressed) I can here a slight backfire in the exhaust an almost popping noise (not loud like a gun shot)kind of a flutter.  When I turn the car off it experiences "run-on" or detonation for about 4 seconds as if it's going to explode.  The temp. is good.  I have a combo temp. / oil pressure gauge made by Smiths from Spitbits.  While driving the needle is half-way between cold and the "N".  When I pull up to a light or to park the car it rises to the "N".  The oil pressure is 70 while at freeway speed dropping to about 20 when at a signal light.  The only thing I can think of is the fuel.  I live in Houston and gas has been an issue since Hurricane Ike.  The car hasn't been on the road the past six weeks while getting a new interior and the first tank of gas in it was regular.  Premium is hard to find.  I tried to retard the timing to 0, but it made no difference.  Do I simply add a fuel additive or am I overlooking something.  Secondly, premium is rated 93 octane and doesn't the high compression engine reguire at least 95?  Does that mean one should add a fuel additive with each fill-up?

Answer
If you had bad gas, you wouldn't have "runs great" and "plenty of power", it would feel like it was missing, and/or you would hear the engine pinging under load.

The symptom sounds like something in the emissions; there are flame traps, air pumps, pcv and egr valves that route vacuum around, and recirculate gases for the engine.  

As a first step, go through all your emissions hoses and make sure you don't have a leak/broken/open line, and that items like the EGR and vacuum retard actually hold vacuum when applied.  If all your emissions gear is original stock, the EGR diaphram may have finally given up the ghost.